Noise Reduction at Source for a Vehicle Using Free Layer Damper 2011-26-0067
Traditional methods of noise control in most application are by using absorption and barrier techniques. These involve brackets & clamps for assembly, carrier material to hold absorbing materials. Usage of absorbing materials which could be high, as this is based on noise control technique by allowing source to produce noise and hence the cost is also higher. Based on the survey, several demerits have been studied in using absorption and barrier noise control techniques in the field of an automobile application. This paper deals with the noise control by using the application of free layer damping technique thereby overcoming the demerits happening in using former techniques, helping better control of noise in the environment and solutions which are more durable.
The methodology followed here before going for the FLD application is identification of noise radiating components which needs to be damped in a system or subsystem. The noise radiating components can be identified by using conventional near source measurement and sound intensity mapping technique. Since all components exhibits the spring-mass-damping behavioural properties, under damped components also needs to be identified through experimental impact testing. The critical frequencies of the components can be extracted through impact testing in free-free condition and also in fixed condition as in actual application. Based on the impact test and sound intensity mapping test results, structure borne frequency contents of the component can be quantified and the surface area can be identified where the damping needs to be improved by using FLD damper.
Free Layer Damper (FLD) is a composite material (mainly combination of rubber and steel) directly fitted onto the surface of the structure using a bonding agent. FLD increases the local rigidity of the surface and it dissipates the vibrational energy in the structure itself thereby controlling the radiation of structure borne noise at resonant frequencies. Energy is dissipated due to the extension and compression of damper under bending stress from the vibrating surface.
Selection of FLD damper for the specific application is an important criteria. Based on the properties of base component (to be damped) and structure-borne frequency content, standard FLD damper can be selected and it should have better loss factor in the frequency range of interest at the operating temperature.
The performance of the damper material can be validated at component level by comparing its damping ratio at critical frequencies in damped and un-damped condition through impact testing and the performance at system level can be validated by comparing the near source sound pressure level at the component in damped and un-damped conditions. Thus this paper demonstrates how the modern low cost materials (FLD) can be used to reduce noise at source and thereby overall vehicle Drive by / Pass-by noise.